Researchers at the California Institute of Technology (Caltech) in Pasadena have directly determined the surface temperature of early Mars for the first time, providing evidence that’s consistent with a warmer and wetter martian past.

By analyzing carbonate minerals in a 4-billion-year-old meteorite that originated near the surface of Mars, the scientists determined that the minerals formed at about 64° Fahrenheit (18° Celsius). “The thing that’s really cool is that 18° is not particularly cold nor particularly hot,” said Woody Fischer from Caltech. “It’s kind of a remarkable result.”

Knowing the temperature of Mars is crucial to understanding the planet’s history — its past climate and whether it once had liquid water. The Mars rovers and orbiting spacecraft have found ancient deltas, rivers, lakebeds, and mineral deposits, suggesting that water did indeed flow. Because Mars now has an average temperature of -81° F (-63° C), the existence of liquid water in the past means that the climate was much warmer then. But what’s been lacking is data that directly points to such a history. “There are all these ideas that have been developed about a warmer, wetter early Mars,” Fischer said. “But there’s precious little data that actually bears on it.” That is, until now.

The finding is just one data point, but it’s the first and only one to date. “It’s proof that early in the history of Mars at least one place on the planet was capable of keeping an Earth-like climate for at least a few hours to a few days,” said John Eiler from Caltech.

To make their measurement, the researchers analyzed one of the oldest known rocks in the world — ALH84001, a martian meteorite discovered in 1984 in the Allan Hills of Antarctica. The meteorite likely started out tens of meters below the martian surface and was blown off when another meteorite struck the area, blasting the piece of Mars toward Earth. The potato-shaped rock made headlines in 1996 when scientists discovered tiny globules in it that looked like fossilized bacteria. But the claim that it was extraterrestrial life didn’t hold up upon closer scrutiny. The origin of the globules, which contain carbonate minerals, remains a mystery.

“It’s been devilishly difficult to work out the process that generated the carbonate minerals in the first place,” Eiler said. But there have been countless hypotheses, he adds, and they all depend on the temperature in which the carbonates formed. Some scientists say the minerals formed when carbonate-rich magma cooled and crystallized. Others have suggested that the carbonates grew from chemical reactions in hydrothermal processes. Another idea is that the carbonates precipitated out of saline solutions. The temperatures required for all these processes range from above 1290° F (700° C) in the first case to below freezing in the last. “All of these ideas have merit,” Eiler said.

Finding the temperature through independent means would, therefore, help narrow down just how the carbonate might have been formed. The researchers turned to clumped-isotope thermometry, a technique developed by Eiler and his colleagues that has been used for a variety of applications, including measuring the body temperatures of dinosaurs and determining Earth’s climate history.

In this case, the team measured concentrations of the rare isotopes oxygen-18 and carbon-13 contained in the carbonate samples. Carbonate is made out of carbon and oxygen, and as it forms, the two rare isotopes may bond to each other. The lower the temperature, the more the isotopes tend to bond. As a result, determining the amount of bonding allows for a direct measurement of temperature.

The temperature the researchers measured — 64 ± 7° F (18 ± 4° C) — rules out many carbonate-formation hypotheses. “A lot of ideas that were out there are gone,” Eiler said. For one, the mild temperature means that the carbonate must have formed in liquid water. “You can’t grow carbonate minerals at 18° other than from an aqueous solution,” he said. The new data also suggests a scenario in which the minerals formed from water that filled the tiny cracks and pores inside rock just below the surface. As the water evaporated, the rock outgassed carbon dioxide, and the solutes in the water became more concentrated. The minerals then combined with dissolved carbonate ions to produce carbonate minerals, which were left behind as the water continued to evaporate.

Could this wet and warm environment have been a habitat for life? Most likely not, the researchers say. These conditions wouldn’t have existed long enough for life to grow or evolve — it would have taken only hours to days for the water to dry up. Still, these results are proof that an Earth-like environment once existed in at least one particular spot on Mars for a short time, the researchers say. What that implies for the global geology of Mars — whether this rock is representative of martian history or is just an isolated artifact — is an open question.

The meteorite called ALH84001 is sliced to show its interior. Found in the Allan Hills ice field in Antarctica in 1984, the 4-billion-year-old rock is one of the oldest in the world. The meteorite likely originated just below the surface of Mars. About 16 million years ago, another meteorite struck the area, blasting it off into space before it landed on Earth about 13,000 years ago. Credit: NASA

Researchers at the California Institute of Technology (Caltech) in Pasadena have directly determined the surface temperature of early Mars for the first time, providing evidence that’s consistent with a warmer and wetter martian past.

By analyzing carbonate minerals in a 4-billion-year-old meteorite that originated near the surface of Mars, the scientists determined that the minerals formed at about 64° Fahrenheit (18° Celsius). “The thing that’s really cool is that 18° is not particularly cold nor particularly hot,” said Woody Fischer from Caltech. “It’s kind of a remarkable result.”

Knowing the temperature of Mars is crucial to understanding the planet’s history — its past climate and whether it once had liquid water. The Mars rovers and orbiting spacecraft have found ancient deltas, rivers, lakebeds, and mineral deposits, suggesting that water did indeed flow. Because Mars now has an average temperature of -81° F (-63° C), the existence of liquid water in the past means that the climate was much warmer then. But what’s been lacking is data that directly points to such a history. “There are all these ideas that have been developed about a warmer, wetter early Mars,” Fischer said. “But there’s precious little data that actually bears on it.” That is, until now.

The finding is just one data point, but it’s the first and only one to date. “It’s proof that early in the history of Mars at least one place on the planet was capable of keeping an Earth-like climate for at least a few hours to a few days,” said John Eiler from Caltech.

To make their measurement, the researchers analyzed one of the oldest known rocks in the world — ALH84001, a martian meteorite discovered in 1984 in the Allan Hills of Antarctica. The meteorite likely started out tens of meters below the martian surface and was blown off when another meteorite struck the area, blasting the piece of Mars toward Earth. The potato-shaped rock made headlines in 1996 when scientists discovered tiny globules in it that looked like fossilized bacteria. But the claim that it was extraterrestrial life didn’t hold up upon closer scrutiny. The origin of the globules, which contain carbonate minerals, remains a mystery.

“It’s been devilishly difficult to work out the process that generated the carbonate minerals in the first place,” Eiler said. But there have been countless hypotheses, he adds, and they all depend on the temperature in which the carbonates formed. Some scientists say the minerals formed when carbonate-rich magma cooled and crystallized. Others have suggested that the carbonates grew from chemical reactions in hydrothermal processes. Another idea is that the carbonates precipitated out of saline solutions. The temperatures required for all these processes range from above 1290° F (700° C) in the first case to below freezing in the last. “All of these ideas have merit,” Eiler said.

Finding the temperature through independent means would, therefore, help narrow down just how the carbonate might have been formed. The researchers turned to clumped-isotope thermometry, a technique developed by Eiler and his colleagues that has been used for a variety of applications, including measuring the body temperatures of dinosaurs and determining Earth’s climate history.

In this case, the team measured concentrations of the rare isotopes oxygen-18 and carbon-13 contained in the carbonate samples. Carbonate is made out of carbon and oxygen, and as it forms, the two rare isotopes may bond to each other. The lower the temperature, the more the isotopes tend to bond. As a result, determining the amount of bonding allows for a direct measurement of temperature.

The temperature the researchers measured — 64 ± 7° F (18 ± 4° C) — rules out many carbonate-formation hypotheses. “A lot of ideas that were out there are gone,” Eiler said. For one, the mild temperature means that the carbonate must have formed in liquid water. “You can’t grow carbonate minerals at 18° other than from an aqueous solution,” he said. The new data also suggests a scenario in which the minerals formed from water that filled the tiny cracks and pores inside rock just below the surface. As the water evaporated, the rock outgassed carbon dioxide, and the solutes in the water became more concentrated. The minerals then combined with dissolved carbonate ions to produce carbonate minerals, which were left behind as the water continued to evaporate.

Could this wet and warm environment have been a habitat for life? Most likely not, the researchers say. These conditions wouldn’t have existed long enough for life to grow or evolve — it would have taken only hours to days for the water to dry up. Still, these results are proof that an Earth-like environment once existed in at least one particular spot on Mars for a short time, the researchers say. What that implies for the global geology of Mars — whether this rock is representative of martian history or is just an isolated artifact — is an open question.